Composite solid electrolytes are promising for solid-state Li batteries, yet how filler-matrix interphases govern ion transport remains unclear. Here we develop a printable ternary electrolyte of EMITFSI ionic liquid, a cross-linkable polymer, and LLZTO, and tune the interphase by ultraviolet ozone (UVO) treatment of LLZTO to reduce oxygen vacancies. UVO-treated LLZTO shows higher lattice oxygen and a more negative surface charge, enhancing Li+ solvation and interfacial ion migration. EIS/DRT shows higher conductivity and lower resistance for LEP/UV (0.88 mS cm-1 at 20 °C) than LEP (0.69 mS cm-1), consistent with COMSOL-predicted uniform current distribution. XPS depth profiling and 3D LIBS indicate a thinner, homogeneous SEI on Li. Thus, LEP/UV full cells retain 92.47% capacity after 300 cycles at 0.2 C, outperforming the unmodified system. This work highlights ceramic-filler surface chemistry control to boost interfacial conduction and cycling stability.